In recent years, networks such as WPAN (Wireless Personal Area Network) and a sensor network, which include a wireless communication terminal device that is compact and consumes low power, are attracting attention. One system that is similar to these is a system which includes an active RF tag that actively transmits a wireless signal.
Such a wireless network includes: a control device which is a base station; and terminal devices which are a plurality of wireless communication terminals. The control device incorporates, in a beacon packet, control information for controlling the wireless network, and periodically broadcasts the control information. Based on the control information, terminal devices communicate with the control device. The communication terminals can use various access methods, and for example, CSMA (Carrier Sense Multiple Access), TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), and SDMA (Space Division Multiple Access) can be used.
Although a terminal device that is used in these wireless networks has a slow transmission speed (from several kbps to several hundred kbps), and a short wireless-signal reaching distance (from several meters to several tens of meters); the terminal device is compact and a battery therein has a low-power-consumption capability that can keep driving the terminal device for several years. Improvements have been implemented in a communication protocol and in a frame format in order to reduce a power consumption of the terminal device; and for example, a wireless network disclosed in patent literature 1 has a configuration that provides an active period for conducting a communication within the wireless network, and a non-active period that allows to enter a sleep mode by not conducting a communication. If a prolonged non-active period can be obtained, a prolonged sleep mode can also be obtained; thus the power consumption can be reduced.
FIG. 24 shows one example of a conventional wireless communication system. The wireless communication system is a wireless network that includes a control device and a terminal device. In the example shown in FIG. 24, a wireless network 400 includes: a control device 401 that controls the wireless network 400; terminal devices 402 to 404, which are wireless communication terminals that conduct wireless communications with the control device 401 by following a control of the control device 401.
FIG. 25 shows a superframe period which is one example of a periodical frame configuration. In FIG. 25, the superframe period includes an active period and a non-active period. The active period is a period for conducting communications between the control device 401 and the terminal devices 402 to 404. The non-active period is a period during which a communication is not conducted, and in this period, each of the terminal devices 402 to 404 can reduce power consumptions by entering a sleep mode.
The control device 401 and the terminal devices 402 to 404, all commonly use the active period. The beginning of the active period is used by the control device 401, and the control device 401 broadcasts a beacon frame. The remainder of the active period is used for communications between the control device 401 and the terminal devices 402 to 404, and, for example, CSMA and the like can be used. Furthermore, the active period can be divided into a plurality of time slots, and the slots can be shared between slot CSMA and TDMA. For example, with IEEE 802.15.4 standard, the first half of the time slots are used for a competitive access by CSMA, and each of the time slots in the second half are assigned to be used by a terminal device to conduct a communication.
The beacon frame includes the control information regarding communication, such as an assignment sequence and the number of these time slots, a length of the active period, a length of the non-active period, a time until a next beacon frame transmission, and the like.
FIG. 26 shows one example of a communication sequence between the control device 401 and the terminal devices 402 to 404. The control device 401 broadcasts a beacon frame 460 at the beginning of an active period 451. The terminal devices 402 to 404 receive the beacon frame 460 and acquire the control information. The control information includes information such as the length of the active period, the length of the non-active period, and the like.
Communications are conducted between the control device 401 and the terminal devices 402 to 404 in the active period 451. For example, a data frame 461 is transmitted from the terminal device 403 to the control device 401, and in response, an ACK (Acknowledgement) frame 462 is transmitted from the control device 401 to the terminal device 403.
The communications between the control device 401 and the terminal devices 402 to 404 are not conducted in a non-active period 452. The control device 401 and the terminal devices 402 to 404 can enter the sleep mode during a time notified in the control information, and can reduce the power consumption. The control device 401 and the terminal devices 402 to 404 return to an operation mode immediately before the end of the non-active period 452, and prepare for the communication in the next active period.
When the non-active period 452 ends, the control device 401 starts the next active period and broadcasts a beacon frame 470. Communications are conducted between the control device 401 and the terminal devices 402 to 404 during an active period 453.
An example shown here is one in which a communication from the terminal device 402 to the control device 401 fails. For example, suppose a case where a data frame 471 is transmitted from the terminal device 402 to the control device 401, but a reception error of this data has been generated at the control device 401. In this case, the control device 401 does not transmit, to the terminal device 402, an ACK frame in response to the data frame 471.
The terminal device 402 will continue waiting for an ACK frame from the control device 401 for a predefined period. When the ACK frame is not returned even after the predefined period has elapsed, the terminal device 402 determines that a transmission of the data frame 471 has failed. Then, the terminal device 402 tries to retransmit the data frame, and transmits a retransmission data frame 472. Here, suppose a case where the retransmitted data frame 472 has been safely received by the control device 401. In this case, the control device 401 transmits, to the terminal device 402, an ACK frame 473 in response to the retransmitted data frame 472.
Subsequently, similar operations are repeated, and communications between the control device 401 and the terminal devices 402 to 404 are conducted.